1. Field of the Invention
This invention relates to a class of vibratory rotation sensor in which the vibrating member is a thin-walled axi-symmetric hemispherical shell, and more specifically pertains to the control and readout electronics for such a sensor.
2. Description of Related Art
A prior art vibratory rotation sensor 10 is illustrated in FIG. 1 in an exploded view, as having an outer member 12, a hemispherical resonator 14, and an inner member 16, all of which are made of fused quartz and are joined together with indium. This particular type of vibratory rotation sensor, which has a vibrating member 14 that is a thin-walled axi-symmetric hemispherical shell, is known as a hemispherical resonator gyro (HRG).
The inertially sensitive element in the HRG is the hemispherical resonator 14, usually a thin-walled 5.8 centimeter diameter bell-shaped object positioned between an outer member 12 and an inner member 16 and supported between the inner and outer members by a stem 26.
The thin-walled axi-symmetric hemispherical shell 14 oscillates in one of its lower-order flexing modes (FIG. 2). The shell resonator flexing mode takes the form of a standing wave. The standing wave, which exists around the rim of the shell resonator 14, is shown in its two extremes, 36 and 38, of its oscillatory deformation in FIG. 2.
The elliptical standing wave contains four anti nodes and four nodes, the anti nodes and nodes being separated from one another by 45 degrees. The rotation sensitivity of the standing wave results from the fact that each mass element of the shell undergoing oscillation acts much like a Foucault pendulum attempting to keep the direction of its linear momentum fixed in inertial space when the shell rotates about its axis. The resulting Coriolis forces, the product of the shell's vibratory motion, and the inertial input rate, cause the standing wave to precess with respect to the shell. The standing wave precession angle is known as the gain of the gyro.
In operation, forces are required to control the standing wave on the hemispherical shell resonator 14. These forces are quasi-electrostatic in nature. In the case of the HRG in FIG. 1, a number of electrodes 22 are metallized on the inside surface 20 of the outer housing 12, which is concentric with the hemispherical shell resonator 14. The outer surface 32 of the shell resonator 14 is metallized so that when the device is assembled, the electrodes in the outer housing 12, together with the surface of the resonator they face, form a series of forcing electrostatic capacitors. Voltages applied to the appropriate combinations of these electrodes control the amplitude of the standing wave and also act to suppress unwanted quadrature effects.
Rotation of the HRG 10 about an axis normal to the plane of the rim 34 of shell resonator 14 causes the standing wave to rotate in the opposite direction with respect to the HRG by an angle proportional to the angle of rotation of the HRG 10. Thus, by measuring the angle of rotation of the standing wave with respect to the HRG 10, one can determine the angle of rotation of the HRG 10.
The vibrational mode of the shell resonator 14 is excited by placing a DC bias voltage on the resonator and an AC voltage on the forcing electrodes 22. The frequency of the AC voltage is usually about twice the resonant frequency of the hemispherical shell resonator 14.
Readout signals from the HRG containing information about the amplitude and location of the standing waves on the shell resonator 14 are also obtained capacitively. The capacitive readout is formed by a metallized interior surface 30 of the shell resonator 14 and a plurality of electrodes 24 which are located on an inner concentric quartz housing held in close proximity to the inner metallized shell resonator 14. Because of the shell's oscillating deformation, the capacitance of each of the electrodes 24 is modulated at the resonator flexing frequency. An electronic readout circuit measures these capacitance changes and hence the location and amplitude of the standing wave is determined.
This HRG construction is inherently highly reliable. Its internal electronics consist solely of passive capacitive electrodes sealed in a vacuum. The capacitive electrodes are formed from metallized quartz and a vacuum dielectric between the metallized electrode surfaces, and hence are extremely reliable.
A more compact HRG design involves the concepts of reverse pickoff for readout and combined control and excitation drivers. Rather than measuring the capacitive changes of electrodes 24 to determine the location and amplitude of the standing wave, a readout signal taken directly from the shell resonator 42 (FIG. 3) will provide this same information and require only a single buffer amplifier 48 for the readout signal, rather than a buffer amplifier for each of the capacitive readout electrodes 24. Moreover, the control functions for the HRG can be combined into one set of drive amplifiers 52, 54, 56 and 58 which are connected to the electrodes 50 of the inside inner member 16, thereby eliminating the need for the outer member 12. Such an HRG design is called a single-sided HRG 40. The control and excitation drive signals can be combined into a single set of amplifiers because these are all signals that are input into the gyro. Moreover, the excitation voltages and the control force voltages are at different frequencies. Because of this characteristic, it is possible to apply them to the same capacitive electrodes 44. This eliminates considerable analog circuitry and the requirement for using an outer member 12. The resulting single-sided HRG, because of its size reduction, has found considerable acceptance in applications where small size is at a premium, as is the case in oil well drilling applications and small space systems.
Additional and more specific details of vibratory rotation sensors can be found in U.S. Pat. No. 4,951,508 issued to Loper, Jr. et al. Aug. 28, 1990, the entire disclosure thereof being incorporated herein.
In inertial measurement applications for both space and subterranean environments, minimum-sized gyro packages are required. Moreover, for subterranean applications such as mining, tunneling, and oil and gas drilling, the gyroscope package must survive temperatures in excess of 150.degree. C., along with high shock and vibration. Packaging becomes a considerable problem when the gyros and the electronics must fit into small diameter (less than two inches) boring and drilling tools.
In applications when multiple gyros are required, and specifically three gyros are packaged into a single compact container with each gyro positioned at 120.degree. spacing, the electronics required for the three gyros is much larger than the gyros themselves.